Ac. Kolbert et al., MECHANOCHEMICAL DEGRADATION OF ETHYLENE-PROPYLENE COPOLYMERS - CHARACTERIZATION OF OLEFIN CHAIN-ENDS, Macromolecules, 29(27), 1996, pp. 8591-8598
We have-examined chain ends in ethylene-propylene (EP) copolymers foll
owing high-temperature shearing in an extruder. New chemical structure
s, not present prior to shearing, corresponding to vinyl, vinylidene,
and vinylene chain ends were observed via H-1 and C-13 NMR and FTIR sp
ectroscopy. A compilation of the various types of olefin chain ends po
ssible via main chain scission of EP was made and their C-13 chemical
shifts were calculated and compared with experimental results. This co
mparison, in combination with the results of DEFT experiments, allowed
the assignment of the olefin region of the C-13 spectrum. The mechani
sm of polymer shearing appears to be hydrogen abstraction followed by
disproportionation via beta-scission of the main chain radicals to for
m olefins. A strong preference was shown for abstraction of methine pr
otons, presumably due to the stability of the tertiary radical interme
diate, and main chain scission at the position beta to a methyl group.